Continuous growth of crystalline materials



May 3, 1966 D. c. BENNETT CONTINUOUS GROWTH 0F CRYSTALLINE MATERIALS Filed Feb. 20, 1963 2 Sheets-Sheet 1 lsob LOW AXIAL HEAT C JONDUCTIVITY RELATIVE TO TRANSVERSE HEAT CONDUCTIVITY Isle ISIbLyISIb '5' H0 He n INVENTOR. DONALD C. BENNETT bmwmmmm May 3, 1966 D. c. BENNETT CONTINUOUS GROWTH OF CRYSTALLINE MATERIALS Filed Feb. 20, 1963 2 Sheets-Sheet 2 TEMPERATURE CENTIGRADE INVENTOR DONALD C. BENNETT BY ATT United States Patent CONTINUGUS GRUWTH 0F CRYSTALLINE MATERIALS Donald C. Bennett, East Brunswick, NJ, assignor to 'Merck & Co., fine, Rahway, N.J., a corporation of New Jersey Filed Feb. 20, 1963, Ser. No. 259,861 Claims. (Cl. 23-273) This invention relates to an apparatus and process for the continuous growth of single and oligo crystalline materials, and more particularly, to an improved apparatus and process for the continuous casting of thermoelectric material in the form of elongated rods.

In the preparation of crystalline material from a molten mass of such material, the shape of the liquid/solid interface is important. Thus, where the interface shape is concave into the liquid melt, there is a tendency for spurious nuclei to form on the liquid side of the interface at or near the confining wall. This results in the production of crystalline material that is unsatisfactory for many purposes. The shape of this liquid/ solid interface is influenced largely by those thermal conditions which in the end determine the direction and rate of heat flow in the crystal-forming portions of the apparatus and in the crystalline material so formed.

An important feature of this invention is that it enables a liquid/solid interface shape that is convex into the liquid melt, to be readily and simply achieved. Thus this invention provides a ready and simple way to avoid, or at least substantially reduce, spurious nucleation at the walls of the chamber or die in which the crystalline material is formed. 4

Another important feature of this invention is that by controlling the liquid/solid interface shape in such ready and simple manner, crystalline rods having the desired grain size and crystal orientation can be made more readily than they can with the use of apparatus and processes heretofore known. When the invention is applied to the production of rods of crystalline thermoelectric material, it has been found that the resulting rods have good thermoelectric properties, including a superior figure of merit.

Other features and advantages of the invention will become apparent as the description thereof proceeds.

In the drawings, FIGURE 1 is a vertical cross sectional view of an apparatus embodying the invention for the continuous casting of thermoelectric material;

FIGURE 2 is an enlarged view of the die portion and associated parts of the apparatus shown in FIGURE 1-;

FIGURE 3 is a view similar to FIGURE 2, but on smaller scale together with a temperature-location graph and certain generalized dimensions to assist in describing the operation of the apparatus in the casting of thermoelectric materials and FIGURE 4 is a vertical cross sectional view of a multiple bore (i.e., 4-hole) die operationally similar to the single bore die shown in FIGURES 13.

Referring to FIGURE 1, the apparatus shown con sists of a furnace generally designated 10 out of the bottom of which is drawn the thermoelectric rod 11 into the quenching pot or heat sink generally designatedlZ.

The furnace 10 includes a quartz cylinder 13 open at the bottom and closed at the top, except that near the top a small diameter inlet tube 14 projects laterally to enable the introduction of an appropriate gas at the top of the charge 15 in the cylinder 13. The bottom of the quartz cylinder 13 rests on a horizontal plate 16, preferably of metal. Around the wall of the quartz cylinder are wrapped the wires 17 of an electrical heater, which may have multiple circuits or heating elements at the lower end in order to providea greater concentration of 3,249,404 Patented May 3, 1966 heat input to the charge 15 in that region than at the upper portion of the charge. Around the outside of the quartz cylinder 13 is thermal insulating material 18. A gas feed tube 20 connects with the quartz inlet tube 14.

The open end of the quartz cylinder is closed by a die 21 having a bore 22 through which the molten charge 15 is withdrawn to form the rod 11. The die 21 is supported upon the horizontal plate 16, where it rests in a pocket 19 formed around the top of a cylindrical opening 23 in the plate 16. The bottom periphery of the die 21 is enlarged, as at 24 to form a shoulder abutting against the bottom edge of quartz cylinder 13.

Covering the top surface of horizontal plate 16 is an annular ring 25 of thermal insulating material. Preferably the inner edge 26 of this annular ring extends to the outer surface of the quartz cylinder 13.

Horizontal plate 16 is preferably cooled by the flow of coolant through metal pipes 27 secured, as by metallic areas 28 (applied by a Welding or soldering process), to the underside of the plate 16.

The quenching pot 12 is positioned beneath the furnace 10 so that the rod 11, after it is drawn out of the die 21 of the furnace, is quenched in the inert quenching medium 30, (preferably a liquid, such as oil) contained in the quenching pot. This quenching pot includes a cylinder 31 open at each end, with an outwardly extending flange 32 at the .top by means of which the pot cylinder 31 is secured, with screws 33, to the underside of horizontal plate 15. The bottom of the pot cylinder 31 has an outwardly extending flange 34 to which is secured, as by screws 35, a bottom plate 36 closing off the bottom of the quenching pot. This bottom plate 36 has a relatively small center opening 37 through which the rod 11 passes out of the bottom of the quenching pot 12. Preferably the bottom plate 36 has a downwardly extending hub 38 through which the opening 37 also extends, with a sealing sleeve 40, of suitable elastic material, having its upper portion tightly gripping the outside of the hub 38 and having its lower portion slidably gripping the rod 11.

The quenching liquid 30 is pumped into the quenching pot 12 through inlet pipe 41, and flows out through the overflow pipe 42. Preferably the quenching liquid, after it leaves the quenching pot, is cooled by suitable means (not shown) before it is pumped back into the quenching pot. In addition, the quenching liquid 30 is preferably cooled while it is contained within the quenching pot 12, this being effected by circulation of a coolant through the helical coil 43 surrounding the rod 11 as it moves through the quenching pot 12. The ends 44 and 45 of this coil 43 pass through suitable openings in the bottom plate 36.

The rod 11 extends downward to suitable means (not shown) that exerts a pull upon the rod of a suitable amount, and at a suitable rate, to cause the rod 11 to be formed in the die 21 from the material forming the charge 15.

The charge 15 may be any suitable mixture of substances which, upon heating to a molten state in the furnace 10, may be drawn through a suitable die at the bottom of the furnace to form the thermoelectric rod 11.

As an illustrative example, the charge 15 may be one producing an n-type thermoelectric rod having a high figure of merit, such as is disclosed in my US. application Serial Number 191,286 filed April 30, 1962. In such case the charge may be a composition of 40 parts bismuth, 56 parts tellurium and 4 parts selenium (the parts being by weight), with an excess of 0.0805 percent of iodine. The alloy of the thermoelectric rod 11 produces thereby consists of about 93 mol percent Bi Te about 7 mol percent Bi Se with an excess of 0.0201 mol percent iodine.

that when these crystals are in this orientation, the rod will possess a high figure of merit, which is one of the desired characteristics of a thermoelectric rod.

I have found that this crystal orientation is affected by the shape of the liquid-solid interface 50 between the liquid charge 15 above it and the solid rod 11 below it. More specifically, I have found that when the shape of this interface 50 is convex into the liquid melt, as illustrated, the grains of the alloys, particularly Bi Te in the resulting thermoelectric rod will be large and columnar with their basal plane parallel to the axis of the rod.

I have also found that the shape ofthe interface 50' may be controlled by making the die 21 of thermally anisotropic material having a markedly lower thermal conductivity in the axial direction of the rod 11 than in the planes perpendicular to the axial direction of the rod 11.

Further, I have found that when the die 21 is of thermally anisotropic material as described above, and the die is so shaped and mounted in the furnace that the flow of heat is directed toward the core of the die throughout its thickness, the interface 50 has a convex shape in which the center of the interface is higher than the periphery of the interface. This is the interface shape which results in the most desirable grain size and orientation of the crystal structure of the thermoelectric rod 11.

Reference is now made to FIGURE 2, in which the die 21 and its associated parts are shown on a larger scale.

The die 21 is made of material which is thermally anisotropic, with the thermal conductivity of the material in the direction of the axis of bore 22 in the die (i.e., in the direction indicated by the arrows adjacent the letter K markedly less than its thermal conductivity in planes perpendicular thereto (i.e., .in the directions indicated by the arrows adjacent the letter K In one such material that has been found satisfactory, the thermal conductivity K in the direction indicated by the arrows adjacent thereto, is of the order of less than one percent of the thermal conductivity K in the planes perpendicular thereto, as indicated by the arrows adjacent the letter K42.

In the construction shown in FIGURES 1-3, the thickness of the die 21 (i.e., the distance indicated as d in FIGURE 2) is preferably greater than the diameter of the bore 22 in the die, and usually of the order of about 1 /2 to 3 times. Most of the die is supported in the furnace 10 above the supporting plate 16. Thus, the

depth d of the pocket 19 of the plate 16 in which the' bottom of die 21 rests, is small in relation to the thickness d of the die, being of the order of one-eighth of the thickness d This depth d is also small in relation to the thickness d of the supporting plate 16, being of the order of one-fith of the thickness d Die 21 has an annular recess or cut-away portion 51 extending upwardly from the bottom face of the die a substantial distance d approximately one-third of the thickness d of the die. This annular recess 51, circling the rod 11 between the rod and the supporting plate 16, acts to impede the flow of heat from the lower portion of the die 21 to the supporting plate 16, which, being cooled, would tend to serve as a heat sink. Thus, the heat in the lower portion of the die is directed toward the rod 11.

The bore 22 in the die 21 is shown as tapered, with its smaller diameter at the top and its larger diameter, 22a, at the bottom. The amount of the taper, when used, is usually relatively small, of the order of about In operation, heater 17 maintains the charge in a molten state. The lower portion of the heater 17 transmits heat to the upper portion of the die 21, which conducts it substantially horizontally, as indicated by the broken lines and arrows 52,. inwardly toward the bore of the die. A minor portion of the heat from the heater 17 is transmitted downward through the die, as this is the direction in which the thermal conductivity of the die (K is considerably lower than the thermal conductivity of the die (K in the horizontal direction. Nevertheless, some heat is transmitted through the die to the lower portion of the die, but there it is directed toward the bore of the die rather than to the supporting plate 16, by virtue of the action of the annular rec ss 51.

With the lower end of rod 11 immersed in the quenching pot 12, and with the top surface of that pot positioned relatively close to the bottom surface of die 21 (this distance d usually being less than the thickness d of the die), the dissipation of the heat transmitted toward the core 22: of the die 21 is to the upper portion of the rod 11 (as indicated by the broken lines andarrows 52), down through the rod 11 (as indicated by the broken lines 53),"to Where the rod is positioned in the quenching pot 12, and from there out from the rod into the quenching pot (as indicated by the broken lines and arrows 54).

The resulting pattern of heat fiow in the die, in which there is a concentration and converging of heat flow to the upper position of the wallof the bore 22 of the die, and from there to the material in the bore of the die, produces an interface 50, between liquid state of the material above the interface and the solid state of the material below it, that is convex upwardly, with the convexity being relatively small. This results in a minimum of nucleationof new grains along the wall of the bore just above the interface, so that there is little tendency for the molten liquid to solidify in small crystals.

.Rather, the molten liquid tends to solidify into large in the rod so that their basal plane is parallel to the axis of the rod. Should the convexity of the interface 50 be large, this will tend to produce a misorientation of these crystals as they form, to the impairment of the desired thermoelectric characteristics of the rod 11.

In starting up the apparatus, a seed rod of the desired composition with its upper end tapered in conformity with the taper of bore 22 in die 21, is inserted up through the quenching pot 12 into the bore 22 to close off the bore, the die thus being in position on horizontal supporting plate 16. The charge (in solid form) is then placed on top of the die 21. Conveniently the charge is a premix of the various'ingredients. The quartz cylinder 13 and its associated heating wires 17 and exterior insulation 18 are then placed over the die 21 and the lower edge of the quartz cylinder rested upon supporting plate 16. The electrical heaters are energized, and in due time the charge becomes molten, and so also does the top part of the seed rod. When the desired temperatures are attained and substantially constantthermal conditions achieved, the starter rod is drawn slowly downward by the rod pulling means (not shown). The molten charge 15 solidifies at the interface 50 to form the solid rod 11, and this rod passes downward through the quenching pot 12 and out the, bottom thereof. Inert gas, such as argon,'is introduced from gas feed tube 20 through tube 14 to the top of the charge under a small pressure. This not only prevents a vacuum from forming on the top of the charge in the quartz cylinder 13 as the charge moves out the bottom of the cylinder, but also assists to a small extent in expelling the charge through the bore of the die.

That the interface 50 is convex upwardly may be determined by examination of a section of the rod taken along a plane containing the longitudinal axis of the rod, the section being in the portion of the rod that was within the bore of the die when the apparatus was first started up, or was later stopped for some reason and then restarted. After etching and polishing the section, a photograph is taken of this portion of the section. The interface at the time the apparatus was started or restarted, will be indicated by the line drawn transversely across the rod joining the points where it is evident that there is an abrupt change in the grain structure of the rod.

FIGURE3 is a temperature-location graph of the temperatures experienced in the operation of an apparatus embodying this invention for the continuous casting of the bismuth telluride alloy referred to in this application.

In the furnace, at the distance d above the top surface of die 22, the temperature of the molten charge was 740 C. Within the bore 22 of the die but above the interface 50, at the distance a" below the top surface of the die, the temperature of the molten charge was 620 C. Within the die 22, below the interface 50, and above the lower surface of the die a distance d that is less than the depth of the annular recess 51, the temperature was 480 C. The temperature of the oil in the quenching pot 12 was 20 C. These temperatures are shown in the graph at the right of FIGURE 3, pposite the corresponding positions in the apparatus.

In this embodiment of the invention die 21 was made of the thermally anisotropic material known as pyrographite. This is synthetic material which exhibits a high degree of anisotropy similar to that of natural graphite single crystals. Pyrographite is made by deposition using the technique of pyrolizing or decomposing carbon-bearing gases. It is described in the May-June 1960 issue of Electronic Progress magazine, Volume IV, Numberfi and was obtained from the Raytheon Company, Lexington, Massachusetts, U.S.A. The thermal conductivity, K in the plane of the die was 3.9 watts/ cm. /C./cm., while the thermal conductivity, K in the plane perpendicular to the die (i.e., in the direction of the axis of the die bore) was 0.0253 watt/ cm. C./ cm. The other constants of the apparatus were as follows:

d =0.50 inch d =0.0625 inch d =0.25 inch d =0.l875 inch d =0.375 inch d =0.25 inch' d =0.125 inch d =0.25 inch d =0.125 inch d =0.50 inch d =1.O30 inch d =0.l53 inch Temperature gradient in the vicinity ofthe interface 50=approximately 640 C. per inch.

Rate of draw of rod:1 inch per hour.

The thermoelectric rod so prepared, of approximately 4 mm. diameter, was from a charge having a composition of 40 parts bismuth, 56 parts tellurium and 4 parts selenium (the parts being weight), with an excess of 0.0805 percent of iodine. The thermoelectric rod had a figure of merit of 2.9 l0 per degree K.

Another thermally anisotropic material which. may be used for die 21 is boron nitride deposited from a vapor phase onto the surface of a heated mandrel, to produce a highly oriented polycrystalline structure having a thermal conductivity, K in the plane of the die of 0.4 calorie per cm. per second per C. per centimeter at 100 C., while the thermal conductivity, K in the plane perpendicular to the die (i.e., in the direction of the axis of the die bore) is 0.004, in the same units as before. This material is available from High Temperature Materials, Inc., of Brighton, Massachusetts, U.S.A., which sells it under the name Boralloy.

While the invention has been shown and described in detail in connection with a die 21 having a single bore or hole 22, the invention is also applicable to multi-bore dies. FIGURE 4 shows such a die, 121, generally similar to die 21 of FIGURES 1-3, but having four bores through which the illustrated rods 11a, 11b and 110 are drawn. The fourth rod is not shown, as it was removed by the section along which the view of FIGURE 4 is necessary, as these are the same, or substantially the same, as those previously described for single bore die 21.

While the invention has been described in connection with the continuous casting of n-type thermoelectric rods, it may be also employed in the continuous casting of p-type thermoelectric rods and other single and oligo crystalline materials,

Also, while the invention has been described with the bottom surface of the die 21 of thermally anisotropic material displaced upwardly a distance d from the top surface of the quenching medium 30, this distance d may be effectively reduced in various ways, and thereby further increase the temperature gradient in the vicinity of the interface 50. For example, when the bottom surface of the die 21 between rod 11 and annular recess 51 is engaged by a liquid-cooled metal surface, the temperature gradient in the die 21 in the vicinity of the interface has been increased to around 1000 degrees centigrade per inch.

What is claimed:

1. In an apparatus for continuous casting of single or large crystalline material in the form of elongated rods wherein the molten charge positioned in a heated chamber is withdrawn from the bottom of the chamber through the bore of a rod-forming die having a longitudinal axis in the direction of travel of the material through the die, and wherein the resulting rod passes through a quenching device positioned below the die, the improvement wherein the rod-forming die is made of thermally anisotropic material positioned so that its thermal conductivity in the direction of the longitudinal axis of the bore of the die is markedly lower than its thermal conductivity in planes perpendicular to said axis.

2. An apparatus as described in claim 1 in which the thermal conductivity of the die in the direction of the longitudinal axis of the bore does not exceed ten percent of the thermal conductivity of the die in planes perpendicular to said axis.

3. In an apparatus for continuous casting of thermoelectric material in the form of elongated rods wherein the molten charge positioned in a heated chamber is withdrawn from the bottom of' the chamber through the bore of a rod-forming die having a longitudinal axis in the direction of travel of the material through the die, and wherein the resulting rod passes through a quenching pot positioned below the die, the improvement wherein the rod-forming die is made of thermally anisotropic material positioned so that its thermal conductivity in the direction of the longitudinal axis of the bore of the die is markedly less than its thermal conductivity in planes perpendicular to said axis.

4. An apparatus as described in claim 3 in which the thermal conductivity of the die in the direction of the longitudinal axis of the bore of the die is of the order of two percent or less of the thermal conductivity of the die in planes perpendicular to said axis.

5. An apparatus as described in claim 3 in which the temperature gradient in the bore of the die in the vicinity of the interface between the molten charge and the solid rod is of the order of 500-1000 degrees centigr-ade per inch.

6. An apparatus as described in claim 3 in which the die has an annular recess surrounding the wall of the die bore and extending upwardly from the bottom face of the die a distance which is part of the thickness of the die to interrupt the flow of heat from the solid rod in the lower portion of the die bore outwardly away from the die bore,

7. An apparatus as described in claim 3 in which the die has a plurality of rod-forming bores arranged substantially uniformly around the periphery of the die, with the die having two annular recesses each extending upwardly from the bottom face a distance which is part of the thickness of the die, one of said annular recesses being positioned radially beyond the outer edges of the plurality of rod-forming bores and the other being positioned radially within the inner edges of the rod-forming bores of the die.

8. An'apparatus as described in claim 3 in which the apparatus includes a metallic plate that supports the heated chamber and rod-forming die, the plate having an opening therein through which the solid rod passes as it moves from the die to the quenching pot, and the plate also having a recess in its upper surface into which the bottom periphery of the die fits, thereby positioning the die relative toithe opening in said plate, said plate also having cooling .means near its periphery so that said plate can withstand the heat to which its inner area, proximate-to the heated chamber and die, is subjected, said die having a recess extending upwardly between the wall of the die bore and the exterior side wall of the die, said recess extending upwardly from the bottom face of the die a distance at least sufficient to impede sig nificantly the flow of heat from the solid rod within the die toward said cooled metallic plate, thereby tending to direct the flow of heat down the solid rod toward the quenching pot.

9. An apparatus as described in claim 8 in which the chamber includes a quartz cylinder into the lower portionof which fits the die, with the cylinder having ele'ctrical heating wire coiled around the outside of the cylinder down nearly to the bottom lip of the quartz cylinder but terminating a short distance above the bottom lip 8" of the cylinder, said upwardly extending recess in said die extending upwardly a distance sufiicient to terminate just below the level of the lowest coil of heating wire on the cylinder, so that said recess impedes significantly the flow of heat from the solid rod within the die toward the cooling means near the periphery of the plate that supports the cylinder and die, without significantly impeding the flow of heat from the lowest coil of the electrical heating wire toward the bore of the die.

10. In an apparatus for continuous casting of single or large crystalline material in the form of elongated rods wherein the' molten charge positioned in a heated chamber is withdrawn from the bottom of the chamber through the bore of a rod-forming die having a longitudinal axis in the direction of travel of the material through the die, and wherein the resulting rod passes through a quenching pot positioned below the die, the improvement wherein means are provided to control the direction of thermal energy flow across the liquid-solid crystallization interface and thereby control the orientation of the crystal structure in the rod, said means comprising a thermally anisotropic material for said rod-forming die, positioned so that the thermal conductivity of the die in directions perpendicular to the longitudinal axis of the bore of the die is at least ten times, and preferably at least fifty times, its thermal conductivity in the direction of the longitudinal axis of the die bore.

References Cited by the Examiner.

UNITED STATES PATENTS 2,789,039 4/1957 Jensen -1 23-301 2,893,847 7/ 1959 SohWciCkePt et al 23273 3,002,824 10/1961 Francois 23301 3,124,489 3/1964 Vogel et. al 1481.6

OTHER REFERENCES Hach et al.: Continuous Casting of Thermoelectric Material, August 28, 1961, from Review of Scientific Instruments, Volume32, pages 1341-1343.

Hannay, Semiconductors, February 27, 1959, Reinhold Publishing Corporation, pages 89-95.

NORMAN YUDKOFF, Primary Examiner.

G. HINES. A. J. ADAMCIK, Assistant Examiners. 

1. IN AN APPARATUS FOR CONTINUOUS CASTING OF SINGLE OR LARGE CRYSTALLINE MATERIAL IN THE FORM OF ELONGATED RODS WHEREIN THE MOLTEN CHARGE POSITIONED IN A HEATED CHAMBER IS WITHDRAWN FROM THE BOTTOM OF THE CHAMBER THROUGH THE BORE OF A ROD-FORMING DIE HAVING A LONGITUDINAL AXIS IN THE DIRECTION OF TRAVEL OF THE MATERIAL THROUGH THE DIE, AND WHEREIN THE RESULTING ROD PASSES THROUGH A QUENCHING DEVICE POSITIONED BELOW THE DIE, THE IMPROVEMENT WHEREIN THE ROD-FORMING DIE IS MADE 